1. Technical Field
The present invention relates to an improvement in a single focus imaging lens which is used as an imaging optical system in various camera including a silver salt camera, specifically, a digital camera, a video camera, a monitoring camera or the like, so as to image a subject image for imaging a still image or a moving image. The present invention also relates to an imaging lens suitable for an imaging device using an electronic imaging device such as a digital camera or a digital video camera, an imaging device such as a camera using such an imaging lens and an information device such as a personal digital assistant having an imaging function.
2. Description of the Related Art
In recent years, a digital still camera and a digital video camera has been in widespread use as an imaging device using an imaging element such as a CCD (charge-coupled device) and a CMOS (complementary metal-oxide semiconductor). A digital camera for use in the imaging of a still image is specifically in widespread use instead of a silver salt camera using a so-called silver salt film.
Since an imaging element for use in this type of imaging device includes a large number of pixels, a high optical performance is required for an imaging lens. Moreover, an imaging device has been increasingly downsized in view of its portability. An imaging device required in the market has a high performance and compactness. Furthermore, since an imaging speed required for photographing has increased, a brighter lens is required for an imaging lens.
It is preferable for a field angle of an imaging lens for a digital camera to be a wide angle that can easily perform photographing like a snapshot. A wide-angle lens corresponding to 28 mm in a focal length converted in a 35 mm photograph (Leica size) is desired.
A typical configuration of a single-focus wide-angle lens includes a retrofocus type lens, for example. This retrofocus type lens which can keep an exit pupil position away from an imaging surface has a principal point back of the entire lens system, so that an asymmetry property is increased by a refractive power arrangement, and the correction of a comatic aberration, distortion and a chromatic aberration of magnification becomes insufficient.
As an optical system having a wide angle, a retrofocus type optical system including a front group having a negative refractive power is known as described in Patent Document 1 (Japanese Patent Publication No. 2942280) and Patent Document 2 (Japanese Patent Publication No. 2991524)
As the optical system having a wide angle, a retrofocus type optical system including a front group having a positive refractive power is also known as described in Patent Document 3 (Japanese Patent Publication No. 3392964) and Patent Document 4 (Japanese Patent Application Publication No. 2010-39088).
Like these optical systems, it is desirable for a compact high performance optical system to include a first lens on the most object side (subject side) having a negative refractive power and to include the last lens having a positive refractive power. Moreover, it is necessary to reduce the number of lenses by using a cemented lens or the like so as to prevent the eccentricity of lenses.
The optical systems described in the above-descried Patent Documents 1-4 will be considered below. The optical system described in Patent Document 1 includes two lens groups of negative and positive, but its entire length is long. For this reason, the distortion is −3%. The optical system described in Patent Document 2 also includes negative and positive two lens groups having a small number of lenses, but its entire length is also long. For this reason, the field curvature remains. The optical system described in Patent Document 3 includes two positive lens groups, but the distortion is about −10%. The optical system described in Patent Document 4 also includes two positive lens groups, but the ratio of the imaging face to the entire length is large, so it is not sufficient for downsizing.
As described above, the optical systems described in Patent Documents 1, 2 are the optical systems each having two lens groups. However, the optical system of Patent Document 1 has a long entire length, resulting in −3% distortion. The optical system described in Patent Document 2 includes a small number of lenses, but its entire length is also long, so that the field curvature can not be sufficiently corrected. The optical system described in each of Patent Documents 3, 4 includes tow positive lens groups. However, the optical system described in Patent Document 3 includes about −10% distortion, which is insufficient for the correction of the aberration. The optical system described in Patent Document 4 has a large ratio of the imaging face to the entire length, so that it can not be sufficiently downsized.